The development of new medical technologies has provided an increasing number of options available to doctors for the diagnosis and treatment of cardiovascular diseases. The availability of such equipment has improved the ability of doctors and surgeons to detect and treat cardiovascular disease. Intravascular imaging technologies have enabled doctors to create and view a variety of images generated by a sensor inserted within a vasculature. Such images complement traditional radiological imaging techniques such as angiography by providing images of the tissue within vessel walls rather than showing a two dimensional lumen image.
Intravascular ultrasound (IVUS) analysis finds particular application to a system and method for quantitative component identification within a vascular object including characterization of tissue. It should be appreciated that while the exemplary embodiment is described in terms of an ultrasonic device, or more particularly the use of IVUS data (or a transformation thereof) to characterize a vascular object, the present invention is not so limited. Thus, for example, using backscattered data (or a transformation thereof) based on ultrasound waves or even electromagnetic radiation (e.g., light waves in non-visible ranges) to characterize any tissue type or composition is within the spirit and scope of the present invention.
Imaging portions of a patient's body provides a useful tool in various areas of medical practice for determining the best type and course of treatment. Imaging of the coronary vessels of a patient by techniques involving insertion of a catheter-mounted probe (e.g., an ultrasound transducer array) can provide physicians with valuable information. For example, the image data indicates the extent of a stenosis in a patient, reveals progression of disease, and helps determine whether procedures such as angioplasty or atherectomy are indicated or whether more invasive procedures are warranted.
In an ultrasound imaging system, an ultrasonic transducer probe is attached to a distal end of a catheter that is carefully maneuvered through a patient's body to a point of interest such as within a coronary artery. The transducer probe in known systems comprises a single piezoelectric crystal element that is mechanically scanned or rotated back and forth to cover a sector over a selected angular range. Acoustic signals are transmitted and echoes (or backscatter) from these acoustic signals are received. The backscatter data is used to identify the type or density of a scanned tissue. As the probe is swept through the sector, many acoustic lines (emanating from the probe) are processed to build up a sector-shaped cross-section image of tissue within the patient. After the data is collected, an image of the blood vessel (i.e., an IVUS image) is reconstructed using well-known techniques. This image is then visually analyzed by a cardiologist to assess the vessel components and plaque content. Other known systems acquire ultrasound echo data using a probe comprising an array of transducer elements.
In a particular application of IVUS imaging, ultrasound data is used to characterize tissue within a vasculature and produce images graphically depicting the content of the tissue making up imaged portions of a vessel. Examples of such imaging techniques for performing spectral analysis on ultrasound echoes to render a color-coded tissue map are presented in Nair et al. U.S. Pat. No. 7,074,188 entitled “System and Method of Characterizing Vascular Tissue” and Vince et al. U.S. Pat. No. 6,200,268 entitled “Vascular Plaque Characterization”, the contents of which are incorporated herein by reference in their entirety, including any references contained therein. Such systems analyze response characteristics of ultrasound backscatter (reflected sound wave) data to identify a variety of tissue types found in partially occluded vessels including: fibrous tissue, fibro-fatty, necrotic core, and dense calcium. An example of a known plaque characterization imaging technique is referred to as “virtual histology” (VH).
When characterizing the response of tissue that has been subjected to ultrasound waves, parameter values are considered at a data point in an imaged field. Based upon response characteristics (e.g., power spectra) of known tissue types, tissue at the data point is assigned to a particular tissue type (e.g. necrotic core). Known systems utilize an integrated backscatter parameter that represents a power response over a frequency band. The integrated backscatter parameter generally represents a measure of total reflected ultrasound power at a particular point within a vasculature over a specified frequency band.
Furthermore, tissue characterization based on integrated backscatter spectral analysis using an IVUS probe-based system generally includes estimating a system transfer function (i.e., spectral content) so that the portion of a recorded signal attributable to system and catheter components (as opposed to the actual backscatter signal from the imaged field within a body) can be removed from IVUS echo signals initially received and recorded by the IVUS system.
Typically, the system transfer function is determined by performing a reference measurement. The reference measurement characterizes the system and catheter signal properties using an appropriate medium such as a specular reflector glass plate. The transfer function is thereafter applied to received image signals to render the bare image signal (with the system signal effects removed).
In some instances, generating a transfer function based on a reference backscatter medium is impractical or not feasible. As a consequence, image signal processing techniques have been developed for spectral analysis-based tissue characterization systems that do not rely on the aforementioned “reference” measurements. Instead, an estimation of a transfer function for a system is rendered based upon blind deconvolution (BDC). As the name suggests, a user is not aware of the actual system transfer function. An example of a known BDC technique is referred to as “iterated window maximization” (IWM).
Homomorphic deconvolution (HDC) has been proposed for purposes of improving spatial resolution in grayscale ultrasound images and has been discussed in U.S. Pat. Nos. 6,443,895 and 7,025,724. Another HDC algorithm is described by Torfinn Taxt in IEEE Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 53, issue 8, pp. 1440-1448 (2006).